Protein surface remodeling

ABSTRACT

Aggregation is a major cause of the misbehavior of proteins. A system for modifying a protein to create a more stable variant is provided. The method involves identifying non-conserved hydrophobic amino acid residues on the surface of a protein, suitable for mutating to more hydrophilic residues (e.g., charged amino acids). Any number of residues on the surface may be changed to create a variant that is more soluble, resistant to aggregation, has a greater ability to re-fold, and/or is more stable under a variety of conditions. The invention also provides GFP, streptavidin, and GST variants with an increased theoretical net charge created by the inventive technology. Kits are also provided for carrying out such modifications on any protein of interest.

RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §120 to and is acontinuation of U.S. patent application, U.S. Ser. No. 13/341,231, filedDec. 30, 2011, which is a continuation of U.S. patent application, U.S.Ser. No. 12/303,047, filed Mar. 9, 2010, which is a national stagefiling under 35 U.S.C. §371 of international PCT application,PCT/US2007/070254, filed Jun. 1, 2007, which claims priority under 35U.S.C. §119(e) to U.S. provisional patent applications, U.S. Ser. No.60/810,364, filed Jun. 2, 2006, and U.S. Ser. No. 60/836,607, filed Aug.9, 2006; each of which is incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with government support under GM065400 awardedby the National Institutes of Health. The government has certain rightsin the invention.

BACKGROUND OF THE INVENTION

Proteins are the workhorses of the cell. Proteins catalyze chemicalreactions, transduce signals in biological systems, provide structuralelements in cells and the extracellular matrix, act as messengers, etc.One of the major causes of misbehavior of proteins is aggregation. Thisis not only a problem in the laboratory but also a problem in manydiseases such as Alzheimer's disease. Aggregation is a particularlyvexing problem when it comes to computationally designed proteins. Forexample, TOP7 is a computationally designed protein with a novel fold. Alonger version of TOP7, TOP7 extended, is very prone to aggregation.TOP7ex is expressed predominantly as insoluble aggregates.

As more proteins are either designed or modified to be used a tools tostudy biological systems or as more proteins wild type or modified areused as therapeutic agents, there needs to be a system for routinelymodifying these proteins to be more stable and/or to preventaggregation.

SUMMARY OF THE INVENTION

The present invention provides a system for modifying proteins to makethem more stable. The invention stems from the recognition thatmodifying the hydrophobic areas on the surface of a protein can improvethe extrathermodynamic properties of the protein. The inventive systemis particularly useful in improving the solubility of a protein ofinterest, improving the protein's resistance to aggregation, and/orimproving the protein's ability to renature. All of these properties areparticularly useful in protein production, protein purification, and theuse of proteins as therapeutic agents and research tools.

In one aspect, the invention provides a method of altering the primarysequence of a protein in order to increase the protein's resistance toaggregation, solubility, ability to refold, and/or general stabilityunder a wide range of conditions. The activity of the modified proteinis preferably approximately or substantially the same as the proteinwithout modification. In certain embodiments, the modified proteinretains at least 50%, 75%, 90%, or 95% of the wild type protein'sactivity. In one embodiments, the method includes the steps of (a)identifying the surface residues of a protein of interest; (b)identifying the particular surface residues that are not highlyconserved among other proteins related to the protein of interest (i.e.,determining which amino acids are not essential for the activity orfunction of the protein); (c) determining the hydrophobicity of theidentified non-conserved surface residues; and (e) replacing at leastone or more of the identified hydrophobic, non-conserved residues withan amino acid that is more polar or is charged at physiological pH. Eachof the above steps may be carried out using any technique, computersoftware, algorithm, paradigm, etc. known in the art. After the modifiedprotein is created, it may be tested for its activity and/or the desiredproperty being sought. In certain embodiments, the modified protein ismore stable. In certain embodiments, the modified protein is lesssusceptible to aggregation. The inventive method typically increases thenet charge (positive or negative) on the protein at physiological pH.

In another aspect, the invention provides a method of altering theprimary sequence of a protein in order to increase the protein'sresistance to aggregation, solubility, ability to refold, and/or generalstability under a wide range of conditions by “supercharging” theprotein. That is, the overall net charge on the modified protein isincreased (either positive charge or negative charge) compared to thewild type protein. Preferably, the activity of the modified protein isapproximately or substantially the same as the protein withoutmodification. In certain embodiments, the method includes the steps of(a) identifying the surface residues of a protein of interest; (b)identifying the particular surface residues that are riot highlyconserved among other proteins related to the protein of interest (i.e.,determining which amino acids are not essential for the activity orfunction of the protein); (c) determining the hydrophilicity of theidentified non-conserved surface residues; and (e) replacing at leastone or more of the identified charged or polar, solvent-exposed,non-conserved residues with a charged amino acid that is charged atphysiological pH. In certain embodiments, to make a negatively charged“supercharged” protein, the residues identified for modification aremutated either to aspartate (Asp) or glutamate (Glu) residues. Incertain other embodiments, to make a positively charged “supercharged”protein, the residues identified for modification are mutated either tolysine (Lys) or arginine (Arg) residues. Each of the above steps may becarried out using any technique, computer software, algorithm, paradigm,etc. known in the art. After the modified protein is created, it may betested for its activity and/or the desired property being sought. Incertain embodiments, the modified protein (“supercharged protein”) ismore stable. In certain embodiments, the modified protein is lesssusceptible to aggregation. The inventive method typically increases thenet charge (positive or negative) on the protein at physiological pH.

The theoretical net charge on over 80% of the proteins catalogued in theProtein Data Bank (PDB) fall within ±10. The modified protein created bythe present invention typically have a net charge less than −10 orgreater than +10. In certain embodiments, the modified protein has a netcharge less than −20 or greater than +20. In certain embodiments, themodified protein has a net charge less than −30 or greater than ±30. Incertain embodiments, the modified protein has a net charge less than −40or greater than +40. In certain embodiments, the modified protein has anet charge less than −50 or greater than +50. The modified proteins areable to fold correctly and retain their biological activity.

Any protein may be modified using the inventive system, and proteinvariants created by the inventive system are considered to be part ofthe present invention, as well as polynucleotides or vectors encodingthe variant protein and cells expressing the variant protein. Theinventive system has been used to create several new variants of greenfluorescent protein (GFP). These variants retain their fluorescence;however, they are more stable than current versions of GFP under a widerange of environments. The inventive GFPs are immune to aggregation evenover long periods of time and in environments that induce aggregationand are capable of refolding into a fluorescent protein even after beingdenatured by boiling. The inventive system has also been used to createnew variants of streptavidin and glutathione-S-transferase (GST). Thesevariants retain their biological activity and remain soluble whenheated. The invention also includes polynucleotide sequences encodingthe inventive GFP, streptavidin, and GST protein sequences, vectorsincluding any of these nucleotide sequences, and cells that include sucha polynucleotide sequence or vector, or express the inventive variants.In certain embodiments, the invention includes bacteria or other cellsthat overexpress an inventive variant. The inventive variants may beused in a variety of biological assays known in the art. For example,supercharged GFPs may be used in any assay that currently uses GFP as areporter protein.

In another aspect, the invention provides other proteins that have beenmodified by the inventive system. These modified proteins preferablyretain a significant portion of their original activity. In certainembodiments, the modified protein retains at least 99%, 98%, 95%, or 90%of the activity of the unmodified version. The modified protein may bemore soluble, resistant to aggregation, have a increased ability torefold, and/or have greater stability under a variety of conditions. Theproteins modified by the inventive system include hydrophobic proteins,recombinant proteins, membrane proteins, structural proteins, enzymes,extracellular proteins, therapeutic proteins (e.g., insulin, cytokines,immunoglobulins, fragments of immunoglobulins, etc.), receptors, cellsignaling proteins, cytoplasmic proteins, nuclear proteins,transcription factors, etc. In certain specific embodiments, theproteins are therapeutic proteins for use in human or veterinarymedicine. In certain embodiments, the proteins are unnatural proteins,for example, computationally designed proteins. In other embodiments,the proteins are hybrid proteins, fusion proteins, altered proteins,mutated proteins, genetically engineered proteins, or any other proteinthat has been altered by the hands of man.

Kits are also provided for the practice of the invention. The kits mayinclude the reagents needed to modify a protein of interest to make itmore resistant to aggregation, increase its ability to renature, orincrease its stability overall. Such kits may include all or some of thefollowing: polynucleotides, computer software, nucleotides, primers,vectors, cell lines, instructions, plates, media, buffers, enzymes,Eppendorf tubes, site-directed mutagenesis kits, etc. Preferably, thekit is conveniently packaged for use in a laboratory setting. Theresearcher typically provides the DNA coding sequence of the protein tobe modified using the inventive technique.

Definitions

“Amino acid”: The term “amino acid” refers to the basic structuralsubunits of proteins. An alpha-amino acid consists of an amino group, acarboxyl group, a hydrogen atom, and a side chain (i.e., R group) allbonded to a central carbon atom. This central carbon atom is referred toas the alpha carbon because it is adjacent to the carboxyl group. Thereare twenty natural amino acids including glycine, alanine, valine,leucine, isoleucine, phenylalanine, tyrosine, trypotphan, cysteine,methionine, serine, threonine, lysine, arginine, histidine, aspartate,glutamate, asparagine, glutamate, and proline. Hydrophobic amino acidsinclude alanine, valine, leucine, isoleucine, and phenylalanine.Aromatic amino acids includes phenylalanine, tyrosine, tryptophan, andhistine. Polar amino acids include tyrosine, cysteine, serine,threonine, lysine, arginine, histidine, aspartate, glutamate,asparagine, and glutamine. Sulfur-containing amino acids includecysteine and methionine. Basic amino acids include lysine, arginine, andhistidine. Acidic amino acids include aspartate and glutamate. Unnaturalamino acids have also been inserted into proteins. In certainembodiments, the twenty natural amino acids are referred to when theterm “amino acid” is used.

“Antibody”: The term “antibody” refers to an immunoglobulin, whethernatural or wholly or partially synthetically produced. All derivativesthereof which maintain specific binding ability are also included in theterm. The term also covers any protein having a binding domain which ishomologous or largely homologous to an immunoglobulin binding domain.These proteins may be derived from natural sources, or partly or whollysynthetically produced. An antibody may be monoclonal or polyclonal. Theantibody may he a member of any immunoglobulin class, including any ofthe human classes: IgG, IgM, IgA, IgD, and IgE.

“Conserved”: The term “conserved” refers nucleotides or amino acidresidues of a polynucleotide sequence or amino acid sequence,respectively, that are those that occur unaltered in the same positionof two or more related sequences being compared. Nucleotides or aminoacids that are relatively conserved are those that are conserved amongstmore related sequences than nucleotides or amino acids appearingelsewhere in the sequences.

“Homologous”: The term “homologous”, as used herein is an art-understoodterm that refers to nucleic acids or proteins that are highly related atthe level of nucleotide or amino acid sequence. Nucleic acids orproteins that are homologous to each other are termed homologues.Homologous may refer to the degree of sequence similarity between twosequences (i.e., nucleotide sequence or amino acid). The homologypercentage figures referred to herein reflect the maximal homologypossible between two sequences, i.e., the percent homology when the twosequences are so aligned as to have the greatest number of matched(homologous) positions. Homology can be readily calculated by knownmethods such as those described in: Computational Molecular Biology,Lesk, A. M., ed., Oxford University Press, New York, 1988; Biocomputing:Informatics and Genome Projects, Smith, D. W., ed., Academic Press, NewYork, 1993; Sequence Analysis in Molecular Biology, von Heinje, G.,Academic Press, 1987; Computer Analysis of Sequence Data, Part I,Griffin, A. M., and Griffin, H. G., eds., Humana Press, New Jersey,1994; and Sequence Analysis Primer, Gribskov, M. and Devereux, J., eds.,M Stockton Press, New York, 1991; each of which is incorporated hereinby reference. Methods commonly employed to determinehomology betweensequences include, but are not limited to those disclosed in Carillo,H., and Lipman, D., SIAM J Applied Math., 48:1073 (1988); incorporatedherein by reference. Techniques for determining homology are codified inpublicly available computer programs. Exemplary computer software todetermine homology between two sequences include, but are not limitedto, GCG program package, Devereux, J., et al., Nucleic Acids Research,12(1), 387 (1984)), BLASTP, BLASTN, and FASTA Atschul, S. F. et al., JMolec. Biol., 215, 403 (1990)).

The term “homologous” necessarily refers to a comparison between atleast two sequences (nucleotides sequences or amino acid sequences). Inaccordance with the invention, two nucleotide sequences are consideredto be homologous if the polypeptides they encode are at least about50-60% identical, preferably about 70% identical, for at least onestretch of at least 20 amino acids. Preferably, homologous nucleotidesequences are also characterized by the ability to encode a stretch ofat least 4-5 uniquely specified amino acids. Both the identity and theapproximate spacing of these amino acids relative to one another must beconsidered for nucleotide sequences to be considered homologous. Fornucleotide sequences less than 60 nucleotides in length, homology isdetermined by the ability to encode a stretch of at least 4-5 uniquelyspecified amino acids.

“Peptide” or “protein”: According to the present invention, a “peptide”or “protein” comprises a string of at least three amino acids linkedtogether by peptide bonds. The terms “protein” and “peptide” may be usedinterchangeably. Inventive peptides preferably contain only naturalamino acids, although non-natural amino acids (i.e., compounds that donot occur in nature but that can be incorporated into a polypeptidechain) and/or amino acid analogs as are known in the art mayalternatively be employed. Also, one or more of the amino acids in aninventive peptide may be modified, for example, by the addition of achemical entity such as a carbohydrate group, a phosphate group, afamesyl group, an isofarnesyl group, a fatty acid group, a linker forconjugation, functionalization., or other modification (e.g., alphaaniindation), etc. In a preferred embodiment, the modifications of thepeptide lead to a more stable peptide (e.g., greater half-life in vivo).These modifications may include cyclization of the peptide, theincorporation of D-amino acids, etc. None of the modifications shouldsubstantially interfere with the desired biological activity of thepeptide. In certain embodiments, the modifications of the peptide leadto a more biologically active peptide.

“Polynucleotide” or “oligonucleotide”: Polynucleotide or oligonucleotiderefers to a polymer of nucleotides. Typically, a polynucleotidecomprises at least three nucleotides. The polymer may include naturalnucleosides (i.e., adenosine, thymidine, guanosine, cytidine, uridine,deoxyadenosine, deoxythymidine, deoxyguanosine, and deoxycytidine),nucleoside analogs (e.g., 2-aminoadenosine, 2-thiothymidine, inosine,pyrrolo-pyrimidine, 3-methyl adenosine, C5-propynylcytidine,C5-propynyluridine, C5-bromouridine, C5-fluorouridine, C5-iodouridine,C5-methylcytidine, 7-deazaadenosine, 7-deazaguanosine, 8-oxoadenosine,8-oxoguanosine, O(6)-methylguanine, and 2-thiocytidine), chemicallymodified bases, biologically modified bases (e.g., methylated bases),intercalated bases, modified sugars (e.g., 2′-fluororibose, ribose,2′-deoxyribose, arabinose, and hexose), and/or modified phosphate groups(e.g., phosphorothioates and 5′-N-phosphoramidite linkages).

“Small molecule”: The term “small molecule,” as used herein, refers to anon-peptide, non-oligomeric organic compound either prepared in thelaboratory or found in nature. Small molecules, as used herein, canrefer to compounds that are “natural product-like,” however, the term“small molecule” is not limited to “natural product-like” compounds.Rather, a small molecule is typically characterized in that it containsseveral carbon-carbon bonds, and has a molecular weight of less than1500, although this characterization is not intended to be limiting forthe purposes of the present invention. In certain other preferredembodiments, natural-product-like small molecules are utilized.

“Stable”: The term “stable” as used herein to refer to a protein refersto any aspect of protein stability. The stable modified protein ascompared to the original wild type protein possesses any one or more ofthe following characteristics: more soluble, more resistant toaggregation, more resistant to denaturation, more resistant tounfolding, more resistant to improper or undesired folding, greaterability to renature, increased thermal stability, increased stability ina variety of environments (e.g., pH, salt concentration, presence ofdetergents, presence of denaturing agents, etc.), and increasedstability in non-aqueous environments. In certain embodiments, thestable modified protein exhibits at least two of the abovecharacteristics. In certain embodiments, the stable modified proteinexhibits at least three of the above characteristics. Suchcharacteristics may allow the active protein to be produced at higherlevels. For example, the modified protein can be overexpressed at ahigher level without aggregation than the unmodified version of theprotein. Such characteristics may also allow the protein to be used as atherapeutic agent or a research tool.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1a to 1 b. Supercharged green fluorescent proteins (GFPs). (FIG.1a ) Protein sequences of GFP variants, with fluorophore-formingresidues highlighted green, negatively charged residues highlighted red,and positively charged residues highlighted blue. GFP-30 (SEQ ID NO:24); GFP-25 (SEQ ID NO: 25); sfGFP (SEQ ID NO: 26); GFP+36 (SEQ ID NO:5); GFP+48 (SEQ ID NO: 27). (FIG. 1b ) Electrostatic surface potentialsof sfGFP (left), GFP(+36) (middle), and GFP(−30) (right), colored from−25 kT/e (red) to +25 kT/e (blue).

FIGS. 2a to 2c . Intramolecular properties of GFP variants. (FIG. 2a )Staining and UV fluorescence of purified GFP variants. Each lane andtube contains 0.2 μg of protein. (FIG. 2b ) Circular dichroism spectraof GFP variants. (FIG. 2c ) Thermodynamic stability of GFP variants,measured by guanidinium-induced unfolding.

FIGS. 3a to 3d . Intermolecular properties of supercharged proteins.(FIG. 3a ) UV-illuminated samples of purified GFP variants (“native”),those samples heated 1 min at 100° C. (“boiled”), and those samplessubsequently cooled for 2 h at 25° C. (“cooled”). (FIG. 3b ) Aggregationof GFP variants was induced with 40% TFE at 25° C. and monitored byright-angle light scattering. (FIG. 3c ) Supercharged GFPs adherereversibly to oppositely charged macromolecules. Sample 1: 6 μg ofGFP(+36) in 30 μl of 25 mM Tris pH 7.0 and 100 mM NaCl. Sample 2: 6 μgof GFP(−30) added to sample 1. Sample 3: 30 μg of salmon sperm DNA addedto sample 1. Sample 4: 20 μg of E. coli tRNA added to sample 1. Sample5: Addition of NaCl to 1 M to sample 4. Samples 6-8: identical tosamples 1, 2, and 4, respectively, except using sfGFP instead ofGFP(+36). All samples were spun briefly in a microcentrifuge andvisualized under UV light. (FIG. 3d ) Enzymatic assays of GST variants.Reactions contained 0.5 mg/mL of GST variant, 20 mMchlorodinitrobenzene, 20 mM glutathione, and 100 mM potassium phosphatepH 6.5. Product formation was monitored at 340 nm, resulting in observedreaction rates (k_(obs)) of 6 min⁻¹ for wild-type GST, 2.2 min⁻¹ forGST(−40), and 0.9 min⁻¹ for GST(−40) after being boiled and cooled.

FIGS. 4a to 4b . (FIG. 4a ) Excitation and (FIG. 4b ) emission spectraof GFP variants. Each sample contained an equal amount of protein asquantitated by chromophore absorbance at 490 nm.

FIG. 5. Biotin-binding activity of streptavidin variants, measured asdescribed previously (Kada et al., Rapid estimation of avidin andstreptavidin by fluorescence quenching or fluorescence polarization.Biochim. Biophys. Acta 1427, 44-48 (1999); incorporated herein byreference) by monitoring binding-dependent of biotin-4-fluorescein(Invitrogen). Protein samples were titrated into 0.3 μMbiotin-4-fluorescein (1340, 100 mM NaCl, 1 mM EDTA, 0.1 mg/mL bovineserum albumin (BSA), 50 mM potassium phosphate 7.5. Quenching offluorescence at 526 nm was measured on a Perkin-Elmer LS50B luminescencespectrometer with excitation at 470 nm. Measurements were normalized tocontrol titrations that contained a 600-fold excess of non-fluorescentbiotin. The three proteins in the bottom of the legend are included asnegative controls.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS OF THE INVENTION

The invention provides a system for modifying proteins to be morestable. The system is thought to work by changing non-conserved aminoacids on the surface of a protein to more polar or charged amino acidresidues. The amino acids residues to be modified may be hydrophobic,hydrophilic, charged, or a combination thereof. Any protein may hemodified using the inventive system to produce a more stable variant.These modifications of surface residues have been found to improve theextrathermodynamic properties of proteins. As proteins are increasinglyused as therapeutic agents and as they continue to be used as researchtools, a system for altering a protein to make it more stable isimportant and useful. Proteins modified by the inventive methodtypically are resistant to aggregation, have an increased ability torefold, resist improper folding, have improved solubility, and aregenerally more stable under a wide range of conditions includingdenaturing conditions such as heat or the presence of a detergent.

Any protein may be modified to create a more stable variant using theinventive system. Natural as well as unnatural proteins (e.g.,engineered proteins) may be modified. Example of proteins that may bemodified include receptors, membrane bound proteins, transmembraneproteins, enzymes, transcription factors, extracellular proteins,therapeutic proteins, cytokines, messenger proteins, DNA-bindingproteins, RNA-binding proteins, proteins involved in signaltransduction, structural proteins, cytoplasmic proteins, nuclearproteins, hydrophobic proteins, hydrophilic proteins, etc. The proteinto be modified may be derived from any species of plant, animal, ormicroorganism. In certain embodiments, the protein is a mammalianprotein. In certain embodiments, the protein is a human protein. Incertain embodiments, the proteins is derived from an organism typicallyused in research. For example, the protein to be modified may be from aprimate (e.g., ape, monkey), rodent (e.g., rabbit, hamster, gerbil),pig, dog, cat, fish (e.g., zebrafish), nematode (e.g., C. elegans),yeast (e.g., Saccharomyces cervisiae), or bacteria (e.g., E. coli).

The inventive system is particularly useful in modifying proteins thatare susceptible to aggregation or have stability issues. The system mayalso be used to modify proteins that are being overexpressed. Forexample, therapeutic proteins that are being produced recombinantly maybenefit from being modified by the inventive system. Such modifiedtherapeutic proteins are not only easier to produce and purify but alsomay be more stable with respect to storage and use of the protein.

The inventive system involves identifying non-conserved surface residuesof a protein of interest and replacing some of those residues with aresidue that is hydrophilic, polar, or charged at physiological pH. Theinventive system includes not only methods for modifying a protein butalso reagents and kits that are useful in modifying a protein to make itmore stable.

The surface residues of the protein to be modified are identified usingany method(s) known in the art. In certain embodiments, the surfaceresidues are identified by computer modeling of the protein. In certainembodiments, the three-dimensional structure of the protein is knownand/or determined, and the surface residues are identified byvisualizing the structure of the protein. In other embodiments, thesurface residues are predicted using computer software. In certainparticular embodiments, Average Neighbor Atoms per Sidechain Atom(AvNAPSA) is used to predict surface exposure. AvNAPSA is an automatedmeasure of surface exposure which has been implemented as a computerprogram. See Appendix A. A low AvNAPSA value indicates a surface exposedresidue, whereas a high value indicates a residue in the interior of theprotein. In certain embodiments, the software is used to predict thesecondary structure and/or tertiary structure of a protein and thesurface residues are identified based on this prediction. In otherembodiments, the prediction of surface residues is based onhydrophobicity and hydrophilicity of the residues and their clusteringin the primary sequence of the protein. Besides in silico methods, thesurface residues of the protein may also be identified using variousbiochemical techniques, for example, protease cleavage, surfacemodification, etc.

Of the surface residues, it is then determined which are conserved orimportant to the functioning of the protein. The identification ofconserved residues can he determined using any method known in the art.In certain embodiments, the conserved residues are identified byaligning the primary sequence of the protein of interest with relatedproteins. These related proteins may be from the same family ofproteins. For example, if the protein is an immunoglobulin, otherimmunoglobulin sequences may be used. The related proteins may also bethe same protein from a different species. For example, the conservedresidues may be identified by aligning the sequences of the same proteinfrom different species. To give but another example, proteins of similarfunction or biological activity may be aligned. Preferably, 2, 3, 4, 5,6, 7, 8, 9, or 10 different sequences are used to determine theconserved amino acids in the protein. in certain embodiments, theresidue is considered conserved if over 50%, 60%, 70%, 75%, 80%, or 90%of the sequences have the same amino acid in a particular position. Inother embodiments, the residue is considered conserved if over 50%, 60%,70%, 75%, 80%, or 90% of the sequences have the same or a similar (e.g.,valine, leucine, and isoleucine; glycine and alanine; glutamine andasparagine; or aspartate and glutamate) amino acid in a particularposition. Many software packages are available for aligning andcomparing protein sequences as described herein. As would be appreciatedby one of skill in the art, either the conserved residues may bedetermined first or the surface residues may be determined first. Theorder does not matter. In certain embodiments, a computer softwarepackage may determine surface residues and conserved residuessimultaneously. Important residues in the protein may also be identifiedby mutagenesis of the protein. For example, alanine scanning of theprotein can be used to determine the important amino acid residues inthe protein. In other embodiments, site-directed mutagenesis may beused.

Once non-conserved surface residues of the protein have been identified,each of the residues is identified as hydrophobic or hydrophilic. Incertain embodiments, the residues is assigned a hydrophobicity score.For example, each non-conserved surface residue may be assigned anoctanol/water logP value. Other hydrophobicity parameters may also beused. Such scales for amino acids have been discussed in: Janin,“Surface and Inside Volumes in Globular Proteins,” Nature 277:491-92,1979; Wolfenden et al., “Affinities of Amino Acid Side Chains forSolvent Water,” Biochemistry 20:849-855, 1981; Kyte et al., “A SimpleMethod for Displaying the Hydropathic Character of a Protein,” J. Mol.Biol. 157:105-132, 1982; Rose et al., “Hydrophobicity of Amino AcidResidues in Globular Proteins,” Science 229:834-838, 1985; Corvette etal., “Hydrophobicity Scales and Computational Techniques for DetectingAmphipathic Structures in Proteins,” J. Mol. Biol, 195:659-685, 1987;Charton and Charton, “The Structure Dependence of Amino AcidHydrophobicity Parameters,” J. Theor. Biol. 99:629-644, 1982; each ofwhich is incorporated by reference. Any of these hydrophobicityparameters may be used in the inventive method to determine whichnon-conserved residues to modify. In certain embodiments, hydrophilic orcharged residues are identified for modification.

At least one identified non-conserved or non-vital surface residue isthen chosen for modification. In certain embodiments, hydrophobicresidue(s) are chosen for modification. In other embodiments,hydrophilic and/or charged residue(s) are chosen for modification. Incertain embodiments, more than one residue is chosen for modification.In certain embodiments, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 of theidentified residues are chosen for modification. In certain embodiments,over 10, over 15, or over 20 residues are chosen for modification. Aswould be appreciated by one of skill in the art, the larger the proteinthe more residues that will need to be modified. Also, the morehydrophobic or susceptible to aggregation or precipitation the proteinis, the more residues will need to be modified. In certain embodiments,multiple variants of the protein, each with different modifications, areproduced and tested to determine the best variant in terms of biologicalactivity and stability.

In certain embodiments, the residues chosen for modification are mutatedinto more hydrophilic residues (including charged residues). Typically,the residues are mutated into more hydrophilic natural amino acids. Incertain embodiments, the residues are mutated into amino acids that arecharged at physiological pH. For example, the residue may be changed toan arginine, aspartate, glutamate, histidine, or lysine. In certainembodiments, all the residues to be modified are changed into the samedifferent residue. For example, all the chosen residues are changed to aglutamate residue. In other embodiments, the chosen residues are changedinto different residues; however, all the final residues may be eitherpositively charged or negatively charged at physiological pH. In certainembodiments, to create a negatively charged protein, all the residues tobe mutated are converted to glutamate and/or aspartate residues. Incertain embodiments, to create a positively charged protein, all theresidues to be mutated are converted to lysine residues. For example,all the chosen residues for modification are asparagine, glutamine,lysine, andlor arginine, and these residues are mutated into aspartateor glutamate residues. To give but another example, all the chosenresidues for modification are aspartate, glutamate, asparagine, and/orglutamine, and these residues are mutated into lysine. This approachallows for modifying the net charge on the protein to the greatestextent.

In other embodiments, the protein may be modified to keep the net chargeon the modified protein the same as on the unmodified protein. In stillother embodiments, the protein may be modified to decrease the overallnet charge on the protein while increasing the total number of chargedresidues on the surface. In certain embodiments, the theoretical netcharge is increased by at least +1, +2, +3, +4, +5, +10, +15, +20, +25,+30, or +35. In certain embodiments, the theoretical net charge isdecreased by at least −1, −2, −3, −4, −5, −10, −15, −20, −25, −30, or−35. In certain embodiments, the chosen amino acids are changed intonon-ionic, polar residues (e.g., cysteine, serine, threonine, tyrosine,glutamine, asparagine).

These modification or mutations in the protein may be accomplished usingany technique known in the art. Recombinant DNA techniques forintroducing such changes in a protein sequence are well known in theart. In certain embodiments, the modifications are made by site-directedmutagenesis of the polynucleotide encoding the protein. Other techniquesfor introducing mutations are discussed in Molecular Cloning: ALaboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch, and Maniatis (ColdSpring Harbor Laboratory Press: 1989); the treatise, Methods inEnzymology (Academic Press, Inc., N.Y.); Ausubel et al. CurrentProtocols in Molecular Biology (John Wiley & Sons, Inc., New York,1999); each of which is incorporated herein by reference. The modifiedprotein is expressed and tested. In certain embodiments, a series ofvariants is prepared and each variant is tested to determine itsbiological activity and its stability. The variant chosen for subsequentuse may be the most stable one, the most active one, or the one with thegreatest overall combination of activity and stability. After a firstset of variants is prepared an additional set of variants may beprepared based on what is learned from the first set. The variants aretypically created and overexpressed using recombinant techniques knownin the art.

The inventive system has been used to created variants of GFP. Thesevariants have been shown to be more stable and to retain theirfluorescence. A GFP from Aequorea victoria is described in GenBankAccession Number P42212, incorporated herein by reference. The aminoacid sequence of this wild type GFP is as follows:

(SEQ ID NO: 1) MSKGEELFTGVVPILVELDGDVNGHKFSVSGEGEGDATYGKLTLKFICTTGKLPVPWPTLVTTFSYGVQCFSRYPDHMKQHDFFKSAMPEGYVQERTIFFKDDGNYKTRAEVKFEGDTLVNRIELKGIDFKEDGNILGHKLEYNYNSHNVYIMADKQKNGIKVNFKIRHNIEDGSVQLADHYQQNTPIGDGPVLLPDNHYLSTQSALSKDPNEKRDHMVLLEFVTAAGITHGMDELYK

Wild type GFP has a theoretical net charge of −7. Using the inventivesystem, variants with a theoretical net charge of −29, −30, −25, +36,+48, and +49 have been created. Even after heating the +36 GFP to 95°C., 100% of the variant protein is soluble and the protein retains ≧70%of its fluorescence.

The amino acid sequences of the variants of GFP that have been createdinclude:

GFP-NEG25 (SEQ ID NO: 2)MGHHHHHHGGASKGEELFTGVVPILVELDGDVNGHEFSVRGEGEGDATEGELTLKFICTTGELPVPWPTLVTTLTYGVQCFSRYPDHMKQHDFFKSAMPEGYVQERTISFKDDGTYKTRAEVKFEGDTLVNRIELKGIDFKEDGNILGHKLEYNFNSHDVYITADKQENGIKAEFEIRHNVEDGSVQLADHYQQNTPIGDGPVLLPDDHYLSTESALSKDPNEDRDHMVLLEFVTAAGIDHGMDELYK GFP-NEG29(SEQ ID NO: 3) MGHHHHHHGGASKGEELFDGEVPILVELDGDVNGHEFSVRGEGEGDATEGELTLKFICTTGELPVPWPTLVTTLTYGVQCFSRYPDHMDQHDFFKSAMPEGYVQERTISFKDDGTYKTRAEVKFEGDTLVNRIELKGIDFKEDGNILGHKLEYNFNSHDVYITADKQENGIKAEFEIRHNVEDGSVQLADHYQQNTPIGDGPVLLPDDHYLSTESALSKDPNEDRDHMVLLEFVTAAGIDHGMDELYK GFP-NEG30(SEQ ID NO: 4) MGHHHHHHGGASKGEELFDGVVPILVELDGDVNGHEFSVRGEGEGDATEGELTLKFICTTGELPVPWPTLVTTLTYGVQCFSDYPDHMDQHDFFKSAMPEGYVQERTISFKDDGTYKTRAEVKFEGDTLVNRIELKGIDFKEDGNILGHKLEYNFNSHDVYITADKQENGIKAEFEIRHNVEDGSVQLADHYQQNTPIGDGPVLLPDDHYLSTESALSKDPNEDRDHMVLLEFVTAAGIDHGMDELYK GFP-POS36)(SEQ ID NO: 5) MGHHHHHHGGASKGERLFRGKVPILVELKGDVNGHKFSVRGKGKGDATRGKLTLKFICTTGKLPVPWPTLVTTLTYGVQCFSRYPKHMKRHDFFKSAMPKGYVQERTISFKKDGKYKTRAEVKFEGRTLVNRIKLKGRDFKEKGNILGHKLAYNFNSHKVYITADKRKNGIKAKFKIRHNVKDGSVQLADHYQQNTPIGRGPVLLPRNHYLSTRSKLSKDPKEKRDHMVLLEFVTAAGIKHGRDERYK GFP-POS42(SEQ ID NO: 6) MGHHHHHHGGRSKGKRLFRGKVPILVELKGDVNGHKFSVRGKGKGDATRGKLTLKFICTTGKLPVPWPTLVTTLTYGVQCFSRYPKHMKRHDFFKSAMPKGYVQERTISFKKDGKYKTRAEVKFEGRTLVNRIKLKGRDFKEKGNILGHKLRYNFNSHKVYITADKRKNGIKAKFKIRHNVKDGSVQLADHYQQNTPIGRGPVLLPRKHYLSTRSKLSKDPKEKRDHMVLLEFVTAAGIKHGRKERYK GFP-POS49(SEQ ID NO: 7) MGHHHHHHGGRSKGKRLFRGKVPILVKLKGDVNGHKFSVRGKGKGDATRGKLTLKFICTTGKLPVPWPTLVTTLTYGVQCFSRYPKHMKRHDFFKSAMPKGYVQERTISFKKDGKYKTRAEVKFKGRTLVNRIKLKGRDFKEKGNILGHKLRYNFNSHKVYITADKRKNGIKAKFKIRHNVKDGSVQLAKHYQQNTPIGRGPVLLPRKHYLSTRSKLSKDPKEKRDHMVLKEFVTAAGIKHGRKERYKAs would be appreciated by one of skill in the art, homologous proteinsare also considered to be within the scope of this invention. Forexample, any protein that includes a stretch of 20, 30, 40, 50, or 100amino acids which are 60%, 70%, 80%, 90%, 95%, or 100% homologous to anyof the above sequences is considered part of the invention. In addition,addition and deletion variants are also contemplated by the invention.In certain embodiments, any GFP with a mutated residue as shown in anyof the above sequences is considered part of the invention. In certainembodiments, the sequence includes 2, 3, 4, 5, 6, 7, 8, 9, 10, or moremutations as shown in any of the sequences above.

Any DNA sequence that encodes the above GFP variants is also includewithin the scope of the invention. Exemplary DNA sequences which encodeeach of the variants above are as follows:

GFP-NEG25 (SEQ ID NO: 8)ATGGGGCATCACCATCATCATCATGGCGGTGCGTCTAAGGGGGAGGAGTTATTTACGGGTGTGGTGCCGATCCTGGTGGAGCTTGATGGCGATGTTAACGGCCATGAATTTTCTGTCCGCGGTGAAGGGGAGGGTGATGCCACGGAAGGGGAGCTGACACTTAAATTTATTTGCACCACCGGTGAACTCCCGGTCCCGTGGCCGACCCTGGTGACCACCCTGACCTACGGCGTTCAATGCTTTTCACGTTATCCGGATCACATGAAGCAACACGACTTCTTTAAAAGCGCGATGCCTGAAGGCTATGTTCAAGAACGTACAATTAGTTTTAAAGATGACGGCACCTACAAGACCCGTGCGGAAGTAAAATTTGAAGGGGACACTTTAGTGAACCGCATCGAGCTGAAAGGGATCGATTTTAAAGAAGATGGGAATATCCTGGGACACAAACTTGAATACAACTTTAATAGTCATGACGTCTATATCACGGCGGACAAACAGGAAAACGGAATTAAGGCAGAATTTGAGATTCGGCATAATGTCGAAGATGGCTCGGTACAGTTGGCTGATCACTATCAGCAGAATACGCCGATTGGAGATGGTCCGGTTTTATTACCAGACGATCACTATCTGTCCACCGAATCCGCCCTGAGCAAAGATCCGAATGAAGACCGGGACCATATGGTTCTGCTGGAATTTGTTACGGCGGCTGGTATTGACCATGGCATGGATGAGCTGTATAAGTAG GFP-NEG29 (SEQ ID NO: 9)ATGGGGCATCACCATCATCATCATGGCGGTGCGTCTAAGGGGGAGGAGTTATTTGATGGTGAAGTGCCGATCCTGGTGGAGCTTGATGGCGATGTTAACGGCCATGAATTTTCTGTCCGCGGTGAAGGGGAGGGTGATGCCACGGAAGGGGAGCTGACACTTAAATTTATTTGCACCACCGGTGAACTCCCGGTCCCGTGGCCGACCCTGGTGACCACCCTGACCTACGGCGTTCAATGCTTTTCACGTTATCCGGATCACATGGACCAACACGACTTCTTTAAAAGCGCGATGCCTGAAGGCTATGTTCAAGAACGTACAATTAGTTTTAAAGATGACGGCACCTACAAGACCCGTGCGGAAGTAAAATTTGAAGGGGACACTTTAGTGAACCGCATCGAGCTGAAAGGGATCGATTTTAAAGAAGATGGGAATATCCTGGGACACAAACTTGAATACAACTTTAATAGTCATGACGTCTATATCACGGCGGACAAACAGGAAAACGGAATTAAGGCAGAATTTGAGATTCGGCATAATGTCGAAGATGGCTCGGTACAGTTGGCTGATCACTATCAGCAGAATACGCCGATTGGAGATGGTCCGGTTTTATTACCAGACGATCACTATCTGTCCACCGAATCCGCCCTGAGCAAAGATCCGAATGAAGACCGGGACCATATGGTTCTGCTGGAATTTGTTACGGCGGCTGGTATTGACCATGGCATGGATGAGCTGTATAAGTAG GFP-NEG30(SEQ ID NO: 10) ATGGGGCATCACCATCATCATCATGGCGGTGCGTCTAAGGGGGAGGAGTTATTTGATGGTGTGGTGCCGATCCTGGTGGAGCTTGATGGCGATGTTAACGGCCATGAATTTTCTGTCCGCGGTGAAGGGGAGGGTGATGCCACGGAAGGGGAGCTGACACTTAAATTTATTTGCACCACCGGTGAACTCCCGGTCCCGTGGCCGACCCTGGTGACCACCCTGACCTACGGCGTTCAATGCTTTTCAGATTATCCGGATCACATGGACCAACACGACTTCTTTAAAAGCGCGATGCCTGAAGGCTATGTTCAAGAACGTACAATTAGTTTTAAAGATGACGGCACCTACAAGACCCGTGCGGAAGTAAAATTTGAAGGGGACACTTTAGTGAACCGCATCGAGCTGAAAGGGATCGATTTTAAAGAAGATGGGAATATCCTGGGACACAAACTTGAATACAACTTTAATAGTCATGACGTCTATATCACGGCGGACAAACAGGAAAACGGAATTAAGGCAGAATTTGAGATTCGGCATAATGTCGAAGATGGCTCGGTACAGTTGGCTGATCACTATCAGCAGAATACGCCGATTGGAGATGGTCCGGTTTTATTACCAGACGATCACTATCTGTCCACCGAATCCGCCCTGAGCAAAGATCCGAATGAAGACCGGGACCATATGGTTCTGCTGGAATTTGTTACGGCGGCTGGTATTGACCATGGCATGGATGAGCTGTATAAGTAG GFP-POS36(SEQ ID NO: 11) ATGGGGCATCATCATCATCACCACGGCGGGGCGTCTAAGGGAGAGCGCTTGTTTCGCGGCAAAGTCCCGATTCTTGTGGAGCTCAAAGGTGATGTAAATGGTCATAAATTTAGTGTGCGCGGGAAAGGGAAAGGAGATGCTACGCGGGGCAAGCTCACCCTGAAATTTATTTGCACAACCGGCAAACTGCCAGTGCCGTGGCCTACATTAGTCACTACTCTGACGTACGGTGTTCAGTGCTTTTCTCGCTATCCCAAACACATGAAACGCCATGATTTCTTCAAGAGCGCGATGCCAAAAGGTTATGTGCAGGAACGCACCATCAGCTTTAAAAAAGACGGCAAATATAAAACCCGTGCAGAAGTTAAATTCGAAGGCCGCACCCTGGTCAACCGCATTAAACTGAAAGGTCGTGACTTCAAAGAGAAAGGTAATATTCTTGGTCACAAACTGCGCTATAATTTCAACTCTCACAAAGTTTATATTACGGCGGATAAACGTAAAAACGGGATTAAAGCGAAATTTAAGATTCGTCATAATGTTAAAGACGGCAGTGTGCAGTTAGCGGATCATTATCAGCAGAATACCCCAATTGGTCGCGGTCCAGTGCTGCTGCCGCGTAACCATTATCTGTCGACCCGCAGCAAACTCAGCAAAGACCCGAAAGAAAAACGTGACCACATGGTATTACTGGAATTTGTGACCGCAGCAGGCATTAAACATGGCCGCGATGAACGTTACAAATAG GFP-POS42(SEQ ID NO: 12) ATGGGCCATCATCATCACCACCACGGCGGCCGCTCAAAAGGTAAACGCTTGTTCCGTGGTAAAGTACCGATCTTAGTGGAGCTCAAAGGGGATGTGAATGGCCATAAGTTCTCGGTTCGTGGCAAAGGTAAGGGAGATGCGACGCGCGGCAAATTAACGCTGAAATTCATTTGTACTACAGGTAAACTGCCGGTGCCATGGCCTACTCTCGTCACCACGTTGACCTATGGGGTTCAATGCTTCAGCCGGTACCCTAAACACATGAACCGCCACGATTTCTTCAAATCGGCGATGCCAAAGGGGTATGTCCAGGAACGCACTATCAGCTTCAAAAAAGACGGTAAGTATAAAACTCGTGCTGAAGTTAAATTCGAAGGACGCACACTGGTAAATCGCATTAAATTGAAGGGGCGCGACTTTAAGGAAAAAGGTAATATCTTAGGTCACAAATTGCGCTACAACTTCAACTCTCATAAAGTTTACATTACAGCAGATAAGCGTAAAAATGGCATCAAAGCGAAATTCAAAATTCGTCACAATGTGAAAGATGGTAGCGTGCAATTAGCCGATCATTACCAGCAGAATACGCCGATCGGTCGCGGCCCAGTACTGTTGCCGCGCAAACATTACTTATCTACCCGGAGTAAACTGTCTAAAGACCCAAAAGAGAAGCGCGACCATATGGTTCTCCTGGAGTTTGTCACCGCCGCCGGAATTAAACACGGCCGCAAAGAGCGCTATAAATAG GFP-POS49(SEQ ID NO: 13) ATGGGCCACCATCATCATCACCACGGGGGACGCTCTAAAGGTAAACGTCTGTTTCGTGGAAAGGTGCCCATTCTGGTTAAACTCAAAGGTGATGTCAACGGCCATAAGTTTTCGGTTCGTGGCAAAGGTAAAGGTGATGCGACGCGCGGGAAATTAACACTGAAATTTATTTGCACAACCGGAAAACTCCCTGTGCCGTGGCCGACTTTGGTGACCACATTAACCTATGGTGTTCAATGCTTCTCACGTTATCCGAAGCATATGAAACGTCATGATTTTTTCAAATCGGCTATGCCGAAAGGTTACGTCCAGGAGCGCACCATCTCATTTAAGAAAGACGGTAAGTATAAAACCCGTGCTGAAGTAAAATTCAAAGGACGCACCCTGGTGAATCGCATTAAACTGAAAGGTCGTGATTTCAAAGAAAAGGGAAATATTTTAGGGCATAAGCTCCGTTATAATTTTAACAGTCATAAGGTGTATATTACCGCTGATAAACGCAAAAACGGAATCAAAGCGAAATTTAAGATCCGTCATAATGTAAAAGATGGCTCAGTCCAACTGGCAAAACATTACCAGCAGAATACCCCGATCGGCCGCGGTCCTGTGCTTCTGCCGCGTAAACACTACTTGTCGACCCGGTCAAAATTGAGTAAAGATCCGAAGGAAAAGCGTGATCACATGGTCTTGAAGGAATTTGTAACTGCAGCAGGTATTAAACACGGGCGCAAAGAACGTTACAAATAG

Polynucleotide sequence homologous to the above sequences are alsowithin the scope of the present invention. In certain embodiments, thepolynucleotide sequence include a stretch of 50, 100, or 150 nucleotidesthat are 60%, 70%, 80%, 90%, 95%, 98%, 99%, or 100% homologous to anyone of the above sequence. The present invention also includes sequencewhere one or more nucleotides is inserted or deleted from one of theabove sequences. Any polynucleotide sequence with a mutation as shown inany of the sequences above is considered part of the invention. Incertain embodiments, the sequence includes 2, 3, 4, 5, 6, 7, 8, 9, 10,or more mutations as shown in any of the sequences above.

The present invention also provides vector (e.g., plasmids, cosmids,viruses, etc.) that comprise any of the inventive sequences herein orany other sequence (DNA or protein) modified using the inventive system.In certain embodiments, the vector includes elements such as promoter,enhancer, ribosomal binding sites, etc. sequences useful inoverexpressing the inventive GFP variant in a cell. The invention alsoincludes cells comprising the inventive sequences or vectors. In certainembodiments, the cells overexpress the variant GFP. The cells may hebacterial cells (e.g., E. coli), fungal cells (e.g., P. pastoris), yeastcells (e.g., S. cerevisiae), mammalian cells (e.g., CHO cells), or humancells.

The inventive system has been used to created variants of streptavidin.These variants have been shown to form soluble tetramers that bindbiotin. The amino acid sequence of this wild type streptavidin is asfollows:

(SEQ ID NO: 28) AAEAGITGTWYNQLGSTFIVTAGADGALTGTYESAVGNAESRYVLTGRYDSAPATDGSGTALGWTVAWKNNYRNAHSATTWSGQYVGGAEARINTQWLLTSGTTEANAWKSTLVGHDTFTKVKPSAASWild type streptavidin has a theoretical net charge of −4. Using theinventive system, variants with a theoretical net charge of −40 and ±52have been created. Even after heating the variants to 100° C., theproteins remained soluble.

The amino acid sequences of the variants of streptavidin that have beencreated include:

SAV-NEG40 (SEQ ID NO: 29)MGHHHHHHGGAEAGITGTWYNQLGSTFIVTAGADGALTGTYESAVGDAESEYVLTGRYDSAPATDGSGTALGWTVAWKNDYENAHSATTWSGQYVGGAEARINTQWLLTSGTTEADAWKSTLVGHDTFTKVEPSAAS SAV-POS52 (SEQ ID NO: 30)MGHHHHHHGGAKAGITGTWYNQLGSTFIVTAGAKGALTGTYESAVGNAKSRYVLTGRYDSAPATKGSGTALGWTVAWKNKYRNAHSATTWSGQYVGGAKARINTQWLLTSGTTKAKAWKSTLVGHDTFTKVKPSAASAs would he appreciated by one of skill in the art, homologous proteinsare also considered to be within the scope of this invention. Forexample, any protein that includes a stretch of 20, 30, 40, 50, or 100amino acids which are 60%, 70%, 80%, 90%, 95%, or 100% homologous to anyof the above sequences is considered part of the invention. In addition,addition and deletion variants are also contemplated by the invention.In certain embodiments, any streptavidin with a mutated residue as shownin any of the above sequences is considered part of the invention. Incertain embodiments, the sequence includes 2, 3, 4, 5, 6, 7, 8, 9, 10,or more mutations as shown in any of the sequences above.

Any DNA sequence that encodes the above streptavidin variants is alsoincluded within the scope of the invention. Exemplary DNA sequenceswhich encode each of the variants above are as follows:

SAV-NEG40 (SEQ ID NO: 31)GGTTCAGCCATGGGTCATCACCACCACCATCACGGTGGCGCCGAAGCAGGTATTACCGGTACCTGGTATAACCAGTTAGGCTCAACCTTTATTGTGACCGCGGGAGCGGACGGCGCCTTAACCGGTACCTACGAATCAGCTGTAGGTGACGCGGAATCAGAGTACGTATTAACCGGTCGTTATGATAGCGCGCCGGCGACTGACGGTAGCGGTACTGCTTTAGGTTGGACCGTAGCGTGGAAGAATGATTATGAAAACGCACATAGCGCAACAACGTGGTCAGGGCAGTACGTTGGCGGAGCTGAGGCGCGCATTAACACGCAGTGGTTATTAACTAGCGGCACCACTGAAGCTGATGCCTGGAAGAGCACGTTAGTGGGTCATGATACCTTCACTAAAGTGGAACCTTCAGCTGCGTCATAATAATGACTCGAGACCTGCA SAV-POS52 (SEQ ID NO: 32)GGTTCAGCCATGGGTCATCACCACCACCATCACGGTGGCGCCAAAGCAGGTATTACCGGTACCTGGTATAACCAGTTAGGCTCAACCTTTATTGTGACCGCGGGAGCGAAAGGCGCCTTAACCGGTACCTACGAATCAGCTGTAGGAAACGCAAAATCACGCTACGTATTAACCGGTCGTTATGATAGCGCGCCGGCGACTAAAGGTAGCGGTACTGCTTTAGGTTGGACCGTAGCGTGGAAGAATAAGTATCGTAATGCGCACAGTGCTACCACTTGGTCAGGGCAGTACGTAGGGGGAGCCAAAGCACGTATCAACACGCAGTGGTTATTAACATCAGGTACCACCAAAGCGAAAGCCTGGAAGAGCACGTTAGTGGGTCATGATACCTTCACTAAAGTGAAACCTTCAGCTGCGTCATAATAATGACTCGAGACCTGCA

Polynucleotide sequence homologous to the above sequences are alsowithin the scope of the present invention. In certain embodiments, thepolynucleotide sequence include a stretch of 50, 100, or 150 nucleotidesthat are 60%, 70%, 80%, 90%, 95%, 98%, 99%, or 100% homologous to anyone of the above sequence. The present invention also includes sequencewhere one or more nucleotides is inserted or deleted from one of theabove sequences. Any polynucleotide sequence with a mutation as shown inany of the sequences above is considered part of the invention. Incertain embodiments, the sequence includes 2, 3, 4, 5, 6, 7, 8, 9, 10,or more mutations as shown in any of the sequences above.

The present invention also provides vector (e.g., plasmids, cosmids,viruses, etc.) that comprise any of the inventive sequences herein orany other sequence (DNA or protein) modified using the inventive system.In certain embodiments, the vector includes elements such as promoter,enhancer, ribosomal binding sites, etc. sequences useful inoverexpressing the inventive streptavidin variant in a cell. Theinvention also includes cells comprising the inventive sequences orvectors. In certain embodiments, the cells overexpress the variantstreptavidin. The cells may be bacterial cells (e.g., E. coli), fungalcells (e.g., P. pastoris), yeast cells (e.g., S. cerevisiae), mammaliancells (e.g., CHO cells), or human cells.

The inventive system has been used to created variants ofglutathione-S-transferase (GST). These variants have been shown toretain the catalytic activity of wild type GST. The amino acid sequenceof this wild type GST is as follows:

(SEQ ID NO: 33) MGHHHHHHGGPPYTITYFPVRGRCEAMRMLLADQDQSWKEEVVTMETWPPLKPSCLFRQLPKFQDGDLTLYQSNAILRHLGRSFGLYGKDQKEAALVDMVNDGVEDLRCKYATLIYTNYEAGKEKYVKELPEHLKPFETLLSQNQGGQAFVVGSQISFADYNLLDLLRIHQVLNPSCLDAFPLLSAYVARLSARPKIKAF LASPEHVNRPINGNGKQWild type GST has a theoretical net charge of +2. Using the inventivesystem, a variant with a theoretical net charge of −40 has been created.This variant catalyzes the addition of glutathione to chloronitrobenzenewith a specific activity only 2.7-fold lower than that of wild type GST.Even after heating the variant to 100° C., the protein remained soluble,and the protein recovered 40% of its catalytic activity upon cooling.

The amino acid sequences of variants of GST include:

GST-NEG40 (SEQ ID NO: 34)MGHHHHHHGGPPYTITYFPVRGRCEAMRMLLADQDQSWEEEVVTMETWPPLKPSCLFRQLPKFQDGDLTLYQSNAILRHLGRSFGLYGEDEEEAALVDMVNDGVEDLRCKYATLIYTDYEAGKEEYVEELPEHLKPFETLLSENEGGEAFVVGSEISFADYNLLDLLRIHQVLNPSCLDAFPLLSAYVARLSARPEIEAF LASPEHVDRPINGNGKQGST-POS50 (SEQ ID NO: 35)MGHHHHHHGGPPYTITYFPVRGRCEAMRMLLADQKQSWKEEVVTMKTWPPLKPSCLFRQLPKFQDGKLTLYQSNAILRHLGRSFGLYGKKQKEAALVDMVNDGVEDLRCKYATLIYTKYKAGKKKYVKKLPKHLKPFETLLSKNKGGKAFVVGSKISFADYNLLDLLRIHQVLNPSCLKAFPLLSAYVARLSARPKIKAF LASPEHVKRPINGNGKQAs would be appreciated by one of skill in the art, homologous proteinsare also considered to be within the scope of this invention. Forexample, any protein that includes a stretch of 20, 30, 40, 50, or 100amino acids which are 60%, 70%, 80%, 90%, 95%, or 100% homologous to anyof the above sequences is considered part of the invention. In addition,addition and deletion variants are also contemplated by the invention.In certain embodiments, any streptavidin with a mutated residue as shownin any of the above sequences is considered part of the invention. Incertain embodiments, the sequence includes 2, 3, 4, 5, 6, 7, 8, 9, 10,or more mutations as shown in any of the sequences above.

Any DNA sequence that encodes the above GST variants is also includedwithin the scope of the invention. Exemplary DNA sequences which encodeeach of the variants above are as follows:

GST-NEG40 (SEQ ID NO: 36)GGTTCAGCCATGGGTCATCACCACCACCATCACGGTGGCCCGCCGTACACCATTACATACTTTCCGGTACGTGGTCGTTGTGAAGCGATGCGTATGTTATTAGCGGACCAGGACCAATCATGGGAAGAAGAAGTAGTGACAATGGAAACCTGGCCGCCGTTAAAGCCTAGCTGTTTATTCCGTCAATTACCGAAGTTTCAGGATGGTGATTTAACCTTATACCAGTCTAACGCGATCTTACGTCATTTAGGTCGCTCATTTGGTTTATACGGTGAAGATGAAGAAGAAGCAGCCTTAGTGGATATGGTGAATGATGGCGTGGAAGACTTACGTTGTAAATACGCGACGTTAATTTACACTGATTATGAAGCCGGTAAAGAGGAGTACGTGGAAGAATTACCTGAACACCTGAAGCCGTTTGAAACATTACTGAGCGAAAATGAAGGAGGTGAGGCGTTCGTAGTTGGTAGCGAAATTAGCTTCGCTGATTATAACTTATTAGACTTATTACGCATTCACCAGGTTTTAAATCCTAGCTGTTTAGACGCTTTCCCGTTACTGAGCGCATATGTAGCGCGCCTGAGCGCCCGTCCGGAAATTGAAGCTTTCTTAGCGTCACCTGAACACGTAGACCGCCCGATTAACGGAAACGGCAAGCAGTAATAATGAGGTACCACCTGCA GST-POS50 (SEQ ID NO: 37)GGTTCAGCCATGGGTCATCACCACCACCATCACGGTGGCCCGCCGTACACCATTACATACTTTCCGGTACGTGGTCGTTGTGAAGCGATGCGTATGTTATTAGCGGACCAGAAACAATCATGGAAAGAAGAAGTAGTGACAATGAAGACCTGGCCGCCGTTAAAGCCTAGCTGTTTATTCCGTCAATTACCGAAGTTTCAGGATGGTAAATTAACCTTATACCAGTCTAACGCGATCTTACGTCATTTAGGTCGCTCATTTGGTTTATACGGTAAGAAGCAGAAAGAAGCAGCCTTAGTGGATATGGTGAATGATGGCGTGGAAGACTTACGTTGTAAATACGCGACGTTAATTTACACTAAATATAAAGCCGGTAAAAAGAAGTACGTGAAAAAATTACCTAAACACCTGAAGCCGTTTGAAACATTACTGAGCAAAAATAAAGGAGGTAAGGCGTTCGTAGTTGGTAGCAAGATTAGCTTCGCTGATTATAACTTATTAGACTTATTACGCATTCACCAGGTTTTAAATCCTAGCTGTTTAAAGGCTTTCCCGTTACTGAGCGCATATGTAGCGCGCCTGAGCGCCCGTCCGAAGATCAAAGCTTTCTTAGCGTCACCTGAACACGTGAAGCGCCCGATTAACGGAAACGGCAAGCAGTAATAATGAGGTACCACCTGCA

The present invention also provides vector (e.g., plasmids, cosmids,viruses, etc.) that comprise any of the inventive sequences herein orany other sequence (DNA or protein) modified using the inventive system.In certain embodiments, the vector includes elements such as promoter,enhancer, ribosomal binding sites, etc. sequences useful inoverexpressing the inventive GST variant in a cell. The invention alsoincludes cells comprising the inventive sequences or vectors. In certainembodiments, the cells overexpress the variant GST. The cells may bebacterial cells (e.g., E. coli), fungal cells (e.g., P. pastoris), yeastcells (e.g., S. cerevisiae), mammalian cells (e.g., CHO cells), or humancells.

The present invention also includes kits for modifying proteins ofinterest to produce more stable variants of the protein. These kitstypically include all or most of the reagents needed create a morestable variant of a protein. In certain embodiments, the kit includescomputer software to aid a researcher in designing the more stablevariant protein based on the inventive method. The kit may also includeall of some of the following: reagents, primers, oligonucleotides,nucleotides, enzymes, buffers, cells, media, plates, tubes,instructions, vectors, etc. The research using the kit typicallyprovides the DNA sequence for mutating to create the more stablevariant. The contents are typically packaged for convenience use in alaboratory.

These and other aspects of the present invention will be furtherappreciated upon consideration of the following Examples, which areintended to illustrate certain particular embodiments of the inventionbut are not intended to limit its scope, as defined by the claims.

EXAMPLES Example 1 Supercharging Proteins Can Impart ExtraordinaryResilience

Protein aggregation, a well known culprit in human disease (Cohen, F.E.; Kelly, J. W., Nature 2003, 426, (6968), 905-9; Chiti, F.; Dobson, C.M., Annu Rev Biochern 2006, 75, 333-66; each of which is incorporatedherein by reference), is also a major problem facing the use of proteinsas therapeutic or diagnostic agents (Frokjaer, S.; Otzen, D. E., Nat RevDrug Discov 2005, 4, (4), 298-306; Fowler, S. B.; Poon, S.; Muff, R.;Chiti, F.; Dobson, C. M.; Zurdo, J., Proc Natl Acad Sci USA 2005, 102,(29), 10105-10; each of which is incorporated herein by reference).Insights into the protein aggregation problem have been garnered fromthe study of natural proteins. It has been known for some time thatproteins are least soluble at their isoelectric point, where they bear anet charge of zero (Loeb, J., J Gen Physiol 1921, 4, 547-555;incorporated herein by reference). More recently, small differences innet charge (±3 charge units) have been shown to predict aggregationtendencies among variants of a globular protein (Chiti, F.; Stefani, M.;Taddei, N.; Ramponi, G.; Dobson, C. M., Nature 2003, 424, (6950), 805-8;incorporated herein by reference), and also among intrinsicallydisordered peptides (Pawar, A. P.; Dubay, K. F.; Zurdo, J.; Chiti, F.;Vendruscolo, M.; Dobson, C. M., J Mol Biol 2005, 350, (2), 379-92;incorporated herein by reference). Together with recent evidence thatsome proteins can tolerate significant changes in net charge (forexample, the finding that carbonic anhydrase retains catalytic activityafter exhaustive chemical acetylation of its surface lysines (Gudiksenet al., J Am Chem Soc 2005, 127, (13), 4707-14; incorporated herein byreference)), these observations led us to conclude that the solubilityand aggregation resistance of some proteins might be significantlyenhanced, without abolishing their folding or function, by extensivelymutating their surfaces to dramatically increase their net charge, aprocess we refer to herein as “supercharging”.

We began with a recently reported state-of-the-art variant of greenfluorescent protein (GFP) called “superfolder GFP” (sfGFP), which hasbeen highly optimized for folding efficiency and resistance todenaturants (Pedelacq et al., Nat Biotechnol 2006, 24, (1), 79-88;incorporated herein by reference). Superfolder GFP has a net charge of−7, similar to that of wild-type GFP. Guided by a simple algorithm tocalculate solvent exposure of amino acids (see Materials and Methods),we designed a supercharged variant of GFP having a theoretical netcharge of +36 by mutating 29 of its most solvent-exposed residues topositively charged amino acids (FIGS. 1a and 1b ). The expression ofgenes encoding either sfGFP or GFP(+36) yielded intenselygreen-fluorescent bacteria. Following protein purification, thefluorescence properties of GFP(+36) were measured and found to be verysimilar to those of sfGFP. Encouraged by this finding, we designed andpurified additional supercharged GFPs having net charges of +48, −25,and −30, all of which were also found to exhibit sfGFP-like fluorescence(FIG. 2a ). All supercharged GFP variants showed circular dichroismspectra similar to that of sfGFP, indicating that the proteins havesimilar secondary structure content (FIG. 2b ). The thermodynamicstabilities of the supercharged GFP variants were only modestly lowerthan that of sfGFP (1.0-4.1 kcal/mol, FIG. 2c and Table 1) despite thepresence of as many as 36 mutations.

Although sfGFP is the product of a long history of GFP optimization(Giepmans et al., Science 2006, 312, (5771), 217-24; incorporated hereinby reference), it remains susceptible to aggregation induced by thermalor chemical unfolding. Heating sfGFP to 100° C. induced its quantitativeprecipitation and the irreversible loss of fluorescence (FIG. 3a ). Incontrast, supercharged GFP(+36) and GFP(−30) remained soluble whenheated to 100° C., and recovered significant fluorescence upon cooling(FIG. 3a ). Importantly, while 40% 2,2,2-trifluoroethanol (TFE) inducedthe complete aggregation of sfGFP at 25° C. within minutes, the +36 and−30 supercharged GFP variants suffered no significant aggregation orloss of fluorescence under the same conditions for hours (FIG. 3b ).

In addition to this remarkable aggregation resistance, supercharged GFPvariants show a strong, reversible avidity for highly chargedmacromolecules of the opposite charge (FIG. 3c ). When mixed together in1:1 stoichiometry, GFP(+36) and GFP(−30) immediately formed a greenfluorescent co-precipitate, indicating the association of foldedproteins. GFP(+36) similarly co-precipitated with high concentrations ofRNA or DNA. The addition of NaCl was sufficient to dissolve thesecomplexes, consistent with the electrostatic basis of their formation.In contrast, sfGFP was unaffected by the addition of GFP(−30), RNA, orDNA (FIG. 3c ).

We next sought to determine whether the supercharging principle couldapply to proteins other than GFP, which is monomeric and has awell-shielded fluorophore. To this end, we applied the superchargingprocess to two proteins unrelated to GFP. Streptavidin is a tetramerwith a total net charge of −4. Using the solvent-exposure algorithm, wedesigned two supercharged streptavidin variants with net charges of −40or +52. Both supercharged streptavidin variants were capable of formingsoluble tetramers that bind biotin, albeit with reduced affinity.

Glutathione-S-transferase (GST), a dimer with a total net charge of +2,was supercharged to yield a dimer with net charge of −40 that catalyzedthe addition of glutathione to chlorodinitrobenzene with a specificactivity only 2.7-fold lower than that of wild-type GST (FIG. 3d ).Moreover, the supercharged streptavidins and supercharged GST remainedsoluble when heated to 100° C., in contrast to their wild-typecounterparts, which, like sfGFP, precipitated quantitatively andirreversibly (Table 1). In addition, GST(−40) recovered 40% of itscatalytic activity upon cooling (FIG. 3d ).

In summary, we have demonstrated that monomeric and multimeric proteinsof varying structures and functions can be “supercharged” by simplyreplacing their most solvent-exposed residues with like-charged aminoacids. Supercharging profoundly alters the intermolecular properties ofproteins, imparting remarkable aggregation resistance and the ability toassociate in folded form with oppositely charged macromolecules like“molecular Velcro.” We note that these unusual intermolecular propertiesarise from high net charge, rather than from the total number of chargedamino acids, which was not significantly changed by the superchargingprocess (Table 1).

In contrast to these dramatic intermolecular effects, the intramolecularproperties of the seven supercharged proteins studied here, includingfolding, fluorescence, ligand binding, and enzymatic catalysis, remainedlargely intact. Supercharging therefore may represent a useful. approachfor reducing the aggregation tendency and improving the solubility ofproteins without abolishing their function. These principles may beparticularly useful in de novo protein design efforts, whereunpredictable protein handling properties including aggregation remain asignificant challenge. In light of the above results of superchargingnatural proteins, it is tempting to speculate that the aggregationresistance of designed proteins could also be improved by biasing thedesign process to increase the frequency of like-charged amino acids atpositions predicted to lie on the outside of the folded protein.

Protein supercharging illustrates the remarkable plasticity of proteinsurfaces and highlights the opportunities that arise from the mutationaltolerance of solvent-exposed residues. For example, it was recentlyshown that the thermodynamic stability of some proteins can be enhancedby rationally engineering charge-charge interactions (Strickler et al.,Biochemistry 2006, 45, (9), 2761-6; incorporated herein by reference).Protein supercharging demonstrates how this plasticity can be exploitedin a different way to impart extraordinary resistance to proteinaggregation. Our findings are consistent with the results of acomplementary study in which removal of all charges from ubiquitin leftits folding intact but significantly impaired its solubility (Loladze etal, Protein Sci 2002, 11, (1), 174-7; incorporated herein by reference).

These observations may also illuminate the modest net-chargedistribution of natural proteins (Knight et al., Proc Natl Acad Sci USA2004, 101, (22), 8390-5; Gitlin et al., Angew Chem Int Ed Engl 2006, 45,(19), 3022-60; each of which is incorporated herein by reference): thenet charge of 84% of Protein Data Bank (PDB) polypeptides, for example,falls within ±10. Our results argue against the hypothesis that high netcharge creates sufficient electrostatic repulsion to force unfolding.Indeed, GFP(+48) has a higher positive net charge than any polypeptidecurrently in the PDB, yet retains the ability to fold and fluoresce.Instead, our findings suggest that nonspecific intermolecular adhesionsmay have disfavored the evolution of too many highly charged naturalproteins. Almost all natural proteins with very high net charge, such asribosomal proteins L3 (+36) and L15 (+44), which bind RNA, orcalsequestrin (−80), which binds calcium cations, associate withoppositely charged species as part of their essential cellularfunctions.

Materials and Methods

Design procedure and supercharged protein sequences. Solvent-exposedresidues (shown in grey below) were identified from published structuraldata (Weber, P. C., Ohlendorf, D. H., Wendoloski, J. J. & Salemme, F. R.Structural origins of high-affinity biotin binding to streptavidin.Science 243, 85-88 (1989); Dirr, H., Reinemer, P. & Huber, R. Refinedcrystal structure of porcine class Pi glutathione S-transferase (pGSTP1-1) at 2.1 A resolution. J Mol Biol 243, 72-92 (1994); Pedelacq, J.D., Cabantous, S., Tran, T., Terwilliger, T. C. & Waldo, G. S.Engineering and characterization of a superfolder green fluorescentprotein. Nat Biotechnol 24, 79-88 (2006); each of which is incorporatedherein by reference) as those having AvNAPSA<150, where AvNAPSA isaverage neighbor atoms (within 10 Å) per sidechain atom. Charged orhighly polar solvent-exposed residues (D, E, R, K, N, and Q) weremutated either to Asp or Glu, for negative-supercharging (red); or toLys or Arg, for positive-supercharging (blue). Additionalsurface-exposed positions to mutate in green fluorescent protein (GFP)variants were chosen on the basis of sequence variability at thesepositions among GFP homologues. The supercharging design process forstreptavidin (SAV) and glutathione-S-transferase (GST) was fullyautomated: residues were first sorted by solvent exposure, and then themost solvent-exposed charged or highly polar residues were mutatedeither to Lys for positive supercharging, or to Glu (unless the startingresidue was Asn, in which case to Asp) for negative supercharging.

SAV(−40) (SEQ ID NO: 29); wtSAV (SEQ ID NO: 28); and SAV(+52) (SEQ ID NO: 30).

GST(−40) (SEQ ID NO: 34); wtGST (SEQ ID NO: 33); and GST (+50) (SEQ ID NO: 35).

Protein expression and purification. Synthetic genes optimized for E.coli codon usage were purchased from DNA 2.0, cloned into a pETexpression vector (Novagen), and overexpressed in E. coli BL21(DE3)pLysSfor 5-10 hours at 15° C. Cells were harvested by centrifugation andlysed by sonication. Proteins were purified by Ni-NTA agarosechrornotography (Qiagen), buffer-exchanged into 100 mM NaCl, 50 mMpotassium phosphate pH 7.5, and concentrated by ultrafiltration(Millipore). All GFP variants were purified under native conditions.Wild-type streptavidin was purchased from Promega. Superchargedstreptavidin variants were purified under denaturing conditions andrefolded as reported previously for wild-type streptavidin (Thompson etal. Construction and expression of a synthetic streptavidin-encodinggene in Escherichia coli. Gene 136, 243-246 (1993); incorporated hereinby reference), as was supercharged GST. Wild-type GST was purified undereither native or denaturing conditions, yielding protein of comparableactivity.

Electrostatic surface potential calculations (FIG. 1b ), Models of −30and +48 supercharged GFP variants were based on the crystal structure ofsuperfolder GFP (Pedelacq et al., Engineering and characterization of asuperfolder green fluorescent protein. Nat Biotechnol 24, 79-88 (2006);incorporated herein by reference). Electrostatic potentials werecalculated using APBS (Baker et al., Electrostatics of nanosystems:application to microtubules and the ribosome. Proc. Natl Acad Sci USA98, 10037-10041 (2001); incorporated herein by reference) and renderedwith PyMol (Delano, W. L., The PyMOL Molecular Graphics System,www.pymol.org (2002); incorporated herein by reference) using a scale of−25 kT/e (red) to +25 kT/e (blue).

Protein staining and UV-induced fluorescence (FIG. 2a ). 0.2 μg of eachGFP variant was analyzed by electrophoresis in a 10% denaturingpolyacrylamide gel and stained with Coomassie brilliant blue dye. 0.2 μgof the same protein samples in 25 mM Tris pH 8.0 with 100 mM NaCl wasplaced in a 0.2 mL Eppendorf tube and photographed under UV light (360nm).

Thermal denaturation and aggregation (FIG. 3a ). Purified GFP variantswere diluted to 2 mg/mL in 25 mM Tris pH 8.0, 100 mM NaCl, and 10 mMbeta-mercaptoethanol (BME), then photographed under UV illumination(“native”). The samples were heated to 100° C. for 1 minute, thenphotographed again under UV illumination (“boiled”). Finally, thesamples were cooled 2 h at room temperature and photographed again underUV illumination (“cooled”).

Chemically induced aggregation (FIG. 3b ). 2,2,2-trifluoroethanol (TFE)was added to produce solutions with 1.5 mg/mL protein, 25 mM Iris pH7.0, 10 mM BME, and 40% TFE. Aggregation at 25° C. was monitored byright-angle light scattering.

Size-exclusion chroniotography (Table 1). The multimeric state of SAVand GST variants was determined by analyzing 20-50 μg of protein on aSuperdex 75 gel-filtration column. Buffer was 100 mM NaCl, 50 rnMpotassium phosphate pH 7.5. Molecular weights were determined bycomparison with a set of monomeric protein standards of known molecularweights analyzed separately under identical conditions.

TABLE 1 Calculated and experimentally determined protein properties.name MW (kD) length (aa) n_(pos) n_(neg) n_(charged) Q_(net) pl ΔG(kcal/mol)^(a) native MW (kD)^(b) % soluble after boiling^(c) GFP (−30)27.8 248 19 49 68 −30 4.8 10.2 n.d. 98 GFP (−25) 27.8 248 21 46 67 −255.0 n.d n.d. n.d. sfGFP 27.8 248 27 34 61 −7 6.6 11.2 n.d.  4 GFP (+36)28.5 248 56 20 76 +36 10.4  8.8 n.d. 97 GFP (+48) 28.6 248 63 15 78 +4810.8  7.1 n.d. n.d. SAV (−40) 14.3 137 5 15 20 −10 5.1 n.d. 55 ± 5(tetramer) 99 wtSAV 13.3 128 8 9 17 −1 6.5 n.d. 50 ± 5 (tetramer)  7 SAV(+52) 14.5 137 16 3 19 +13 10.3 n.d. 55 ± 5 (tetramer) 97 GST (−40) 24.7217 17 37 54 −20 4.8 n.d. 50 ± 5 (dimer) 96 wtGST 24.6 217 24 23 47 +17.9 n.d. 50 ± 5 (dimer)  3 GST (+50)^(d) 24.7 217 39 14 53 +25 10.0 n.d.n.d. n.d. n_(pos), number of positively charged amino acids (permonomer) n_(neg), number of negatively charged amino acids n_(charged),total number of charged amino acids Q_(net), theroretical net charge atneutral pH pI, calculated isoelectric point n.d., not determined^(a)measured by guanidinium denaturation (FIG. 2c). ^(b)measured bysize-exclusion chromatography. ^(c)percent protein remaining insupernatant after 5 min at 100° C., cooling to 25° C., and briefcentrifugation. ^(d)protein failed to express in E. coli.

Other Embodiments

Those of ordinary skill in the art will readily appreciate that theforegoing represents merely certain preferred embodiments of theinvention. Various changes and modifications to the procedures andcompositions described above can be made without departing from thespirit or scope of the present invention, as set forth in the followingclaims.

1.-78. (canceled)
 79. A supercharged protein variant of a wild-typeprotein, wherein the supercharged protein variant comprises a modifiedprimary amino acid sequence as compared to the wild-type sequence,resulting in a theoretical net charge on the supercharged proteinvariant of +10 to +52 at physiological pH, wherein the wild-type proteinis selected from the group consisting of enzymes, transcription factors,DNA-binding proteins, and RNA-binding proteins.
 80. The superchargedprotein of claim 1, wherein the net charge at physiological pH of thesupercharged protein variant is increased by at least +3 as compared tothe wild-type sequence.
 81. The supercharged protein of claim 80,wherein the net charge at physiological pH of the supercharged proteinvariant is increased by at least +25 as compared to the wild-typesequence.
 82. The supercharged protein variant of claim 1, wherein thetheoretical net charge at physiological pH of the supercharged proteinis within the range of +52 to +20.
 83. The supercharged protein variantof claim 82, wherein the theoretical net charge at physiological pH ofthe supercharged protein is within the range of +52 to +30.
 84. Thesupercharged protein variant of claim 1, wherein the superchargedprotein variant retains at least 50% of the activity of the wild-typeprotein.
 85. The supercharged protein variant of claim 84, wherein thesupercharged protein variant retains at least 90% of the activity of thewild-type protein.
 86. The supercharged protein variant of claim 1,wherein the modified primary amino acid sequence comprises replacing aplurality of non-conserved, surface residues with a natural amino acidresidue that is positively charged at physiological pH; and whereinnon-conserved, surface residues are identified by comparing the aminoacid sequence of the protein with at least one other amino acid sequenceof the protein from the same protein family or a different species,wherein a residue is non-conserved if less than or equal to 50% of theamino acid sequences have the same amino acid sequence in a particularposition.
 87. The supercharged protein variant of claim 1, wherein themodified primary amino acid sequence of the supercharged protein variantcomprises a replacement of at least five surface residues of thewild-type protein with a different residue.
 88. The supercharged proteinvariant of claim 87, wherein the modified primary amino acid sequence ofthe supercharged protein variant comprises a replacement of at leastfive surface residue of the wild-type protein with a lysine, histidine,or arginine residue.
 89. The supercharged protein variant of claim 1,wherein the wild type protein is a monomeric protein.
 90. Thesupercharged protein variant of claim 1, wherein the wild type proteinis a multimeric protein.
 91. The supercharged protein variant of claim1, wherein the wild type protein is an enzyme.
 92. The superchargedprotein variant of claim 1, wherein the wild type protein is atranscription factor.
 93. The supercharged protein variant of claim 1,wherein the wild type protein is a DNA-binding protein.
 94. Thesupercharged protein variant of claim 1, wherein the wild type proteinis a RNA-binding protein.
 95. The supercharged protein variant of claim1, wherein the variant is a fusion protein.
 96. The supercharged proteinvariant of claim 91, wherein the fusion protein comprises a linker. 97.A complex comprising the supercharged protein variant of claim 1 and anoppositely charged macromolecule.
 98. A composition comprising thesupercharged protein variant of claim 1.